This invention pertains to improved balances which utilize an elastic property of glass to determine mass. More particularly, it pertains to balances for which the resolution shows a low or minimal sensitivity to changes of temperature.
There are a number of approaches for the accurate measurement of mass. Each approach in one way or another relates the mass (m) through a force (F) by means of acceleration (a), i.e. EQU F=ma.
In most laboratory balances, acceleration is provided by gravity. Inertial balances, on the other hand, provide the acceleration by mechanical (usually oscillating) motion. For gravity-dependent balances, masses are measured by counterweights or through deformation leading to a restoring force in a supporting member. These restoring forces are produced by elastic deformation or torsional moments in supporting members. These quantities result from the stiffness of the members which are characterized by their elastic or Young's modulus. For an accurate measurement, therefore, it is important that Young's modulus remain essentially constant, and for measurement of very small masses or for extremely accurate measurement of mass the constancy of Young's modulus is critical. In virtually all materials, however, the value of Young's modulus changes with temperature. In the determination of very small masses, or for high resolution, unless the temperature is carefully controlled this effect can result in significant errors. Constant temperature can be achieved and maintained only within certain limits even with considerable effort and expense, and the equipment required also contributes to operational inconvenience. In some applications adequate temperature control is nearly impossible, which results in a severe degradation or loss of resolution. These problems can be significantly reduced by the use of material which has low or minimal temperature dependence of Young's modulus over the nominal temperature operating range of the system.
Balances are conveniently divided into the two types mentioned above; gravitational and inertial, almost all of which may be designed to use the elastic properties of glass as the measuring means. In the case of counterweight type balances, the consideration of variability of Young's modulus does not apply; however for other gravitational or inertial balances, the constancy of Young's modulus becomes a significant or even critical factor when determining masses with high resolution. In a system which utilizes the bending or vibration of a member to determine mass, the restoring force, which together with mass determines frequency, is proportional to Young's modulus. A change in Young's modulus results in a change in the amount of displacement or frequency which can be misinterpreted as a change in mass. Consequently, the use of a material with minimal temperature dependence of Young's modulus becomes critical.